In a fuel cell system, a fuel cell of a solid polymer type including a solid polymer film having proton conductivity in an electrolytic layer is mounted. When the solid polymer film of this fuel cell is in a wetted state, the film exhibits high proton conductivity, so that it is important to keep the solid polymer film in the wetted state in order to efficiently generate a power. In order to keep such a solid polymer film in the wetted state, a water exchange type humidifier (hereinafter referred to simply as the “humidifier”) is utilized which performs water exchange between an oxidizing off gas to be discharged from a cathode side of the fuel cell and an oxidizing gas to be supplied to the cathode side of the fuel cell via a water vapor transmission film.
Here, it is known that an internal resistance of the fuel cell influences wettability of the electrolytic film in the fuel cell. When the fuel cell has a small internal water content and the electrolytic film dries (so-called dry-up), the internal resistance increases, and an output voltage of the fuel cell drops. On the other hand, when the fuel cell includes an excessively large internal water content (so-called flooding), electrodes of the fuel cell are covered with the water content, and the output voltage drops.
As a method of efficiently operating such a fuel cell, a method is proposed in which a cell voltage of the fuel cell is monitored, and an amount of an oxidizing gas to be supplied is controlled based on a measured value of the cell voltage (e.g., see the following patent document 1).    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-119139